Our school used Pintos, a derivative of Nachos. Taking the OS course there was an eye-opener for a student as technically immature and inexperienced as I was at that time, but it was also a tremendous learning experience. I've always wished I could go back and retake that course, the 'right way'...with these resources, though, maybe there's a better way.
https://en.wikipedia.org/wiki/Not_Another_Completely_Heurist...
3rd year OS class we had to use Nachos as our base OS and build your usual file management, pipes etc. functionality on top of it.
Really taught you how an OS works.